1. Field of the Invention
The present invention relates to an information recording medium that is capable of recording, erasing, rewriting and reproducing information optically or electrically, as well as to a method for manufacturing the same.
2. Related Background Art
As conventional information recording media, phase-changing information recording media are known that record, erase and rewrite information using the phenomenon that a phase change between a crystalline phase and an amorphous phase occurs in a recording layer (phase-changing material layer) that is made of a phase-changing material. Among such phase-changing information recording media, there are optical phase-changing information recording media, with which information can be recorded, erased, rewritten and reproduced using a laser beam. In these optical phase-changing information recording media heat generated by irradiation of a laser beam causes a phase change between the crystalline phase and the amorphous phase in the phase-changing material of the recording layer, and differences in the reflectance of the crystalline phase and the amorphous phase are detected and read out as information. In rewritable optical phase-changing information recording media, which allow the erasing and rewriting of information, the initial state of the recording layer is ordinarily the crystalline phase, and to record information, the recording layer is melted through irradiation of a laser beam of high power (recording power) and rapidly cooled down, turning the portions on which the laser beam was irradiated (laser beam irradiation portions) into the amorphous phase. On the other hand, to erase information, the recording layer is heated by irradiating a laser beam of a power that is lower than during recording (erasing power) and slowly cooled, thus turning the laser beam irradiation portions into the crystalline phase. Consequently, in rewritable optical phase-changing information recording media, it is possible to record new information while erasing recorded information, that is, to rewrite information, by irradiating onto the recording layer a laser beam whose power is modulated between a high power level and a low power level. Moreover, in write-once optical phase-change information recording media in which information can be recorded once, but cannot be erased or rewritten, the recording layer is generally in the amorphous phase in its initial state. To record information on such a recording medium, the recording layer is heated by irradiating a laser beam with high power (recording power), and then slowly cooled, thus turning the laser beam irradiation portions into the crystalline phase.
There are also electrical phase-change information recording media, in which information is recorded by phase-changing the phase-changing material of the recording layer through joule heat that is generated by applying electric energy (for example an electric current) instead of irradiating a laser beam. In such electrical phase-change information recording media, the phase-changing material of the recording layer undergoes a phase change between a crystalline phase (low resistance) and an amorphous phase (high resistance) due to joule heat that is generated by applying a current, for example, and information is read out by detecting the difference in electric resistance between the crystalline phase and the amorphous phase.
An example of optical phase-change information recording media is the 4.7 GB DVD-RAM (digital versatile disk—random access memory) proposed by the inventors (see for example JP H10-275360A). Like the information recording medium 12 shown in FIG. 12, this 4.7 GB DVD-RAM includes, from the side from which a laser beam 11 is irradiated, a substrate 1, and an information layer 100 having a seven-layer structure including a first dielectric layer 2, a first interface layer 3, a recording layer 4, a second interface layer 5, a second dielectric layer 6, a light-absorbing correction layer 7, and a reflective layer 8, layered in this order on top of the substrate 1.
The first dielectric layer 2 and the second dielectric layer 6 have the optical function of increasing the optical absorption efficiency by adjusting the optical distance and increasing the signal intensity by increasing the change of the reflectance between when the recording layer 4 is in the crystalline phase and when it is in the amorphous phase, and furthermore have the thermal function of thermally insulating the recording layer 4, which becomes hot during recording, from the substrate 1 and the dummy substrate 10, which are easily damaged by heat. Conventionally, a mixture of 80 mol % ZnS and 20 mol % SiO2 (referred to below as (ZnS)80(SiO2)20 (mol %) or (ZnS)80(SiO2)20; the same notation also is used for other mixtures) is used for the dielectric layers. This mixture is a superior dielectric material, which is transparent, has a high refractive index, a low thermal conductivity, and a good thermal insulation, and has favorable mechanical characteristics and moisture resistance. It should be noted that the film thickness of the first dielectric layer 2 and the second dielectric layer 6 can be determined strictly by calculation with the matrix method, such that the following conditions are satisfied: the change of the reflected light amount between the crystalline phase and the amorphous phase of the recording layer 4 is large, and the optical absorption at the recording layer 4 is large. By using for the recording layer 4 a high-speed crystallizing material including Ge—Sn—Sb—Te, in which some of the Ge in the pseudo-binary system phase-changing material GeTe—Sb2Te3 obtained by mixing the compounds GeTe and Sb2Te3, is substituted with Sn, it is possible to realize not only initial recording/rewriting properties, but also excellent archival characteristics (indicating whether a recorded signal can be reproduced after storage for a long time), and archival overwrite characteristics (indicating whether a recorded signal can be erased or rewritten after storage for a long time).
The first interface layer 3 and the second interface layer 5 have the function of preventing substance migration that otherwise may occur between the first dielectric layer 2 and the recording layer 4 or between the second dielectric layer 6 and the recording layer 4. Here, “substance migration” refers to the phenomenon that if (ZnS)80(SiO2)20 (mol %) is used for the first dielectric layer 2 and the second dielectric layer 6, sulfur in the (ZnS)80(SiO2)20 (mol %) diffuses into the recording layer when repeatedly writing/rewriting the recording layer 4 through irradiation of the laser beam 11. When sulfur diffuses into the recording layer, the repeated rewriting properties deteriorate. In order to prevent this deterioration of the repeated rewriting properties, it is advantageous to use a nitride including Ge for the first interface layer 3 and the second interface layer 5.
With the above-described technology, excellent repeated rewriting properties and high reliability were achieved, and a 4.7 GB DVD-RAM was proposed and brought to market.
Various technologies have been studied to increase the capacity of information recording media even further. For example, for optical phase-changing information recording media, a technology has been studied in which high-density recording is performed by reducing the spot diameter of the laser beam by using a bluish-purple laser having a wavelength that is shorter than that of conventional red lasers, or by making the substrate on the side from which the laser beam is irradiated thin and using an objective lens having a large numerical aperture (NA). When recording with smaller spot diameters, the time for which the laser beam is irradiated onto the recording layer becomes relatively short. Therefore, in order to enable crystallization with shorter times, it is necessary to make the recording layer from a more readily crystallizing material, or to provide a layer with a high crystallization enhancing effect adjacent to the recording layer.
Furthermore, technologies have been studied by which the recording capacity is doubled by using optical phase-changing information recording media having two information layers (in the following also referred to as “double-layer optical phase-changing information recording media”) and recording and reproducing information on two information layers with a laser beam that is irradiated from one side (see for example JP 2000-36130A and JP 2002-144736A). In double-layer optical phase-changing information recording media, in order to record/reproduce the information layer that is located further away from the side from which the laser beam is incident (referred to as “second information layer” in the following), a laser beam is used that passes through the information layer located closer to the side from which the laser beam is incident (referred to as “first information layer” in the following), so that in the first information layer, the recording layer is made thin and its transmittance is increased. However, when the recording layer becomes thin, the number of crystal nuclei that are formed when the recording layer is crystallized is reduced, and the distance over which atoms can move is shortened. Therefore, the thinner the recording layer is, the more difficult it becomes to form the crystalline phase becomes (i.e. the crystallization speed decreases). Consequently, in a first information layer having a thin recording layer, it is necessary to make the recording layer from a material with better crystallization capability or to provide a layer with a high crystallization enhancing effect adjacent to the recording layer.
Furthermore, when the time it takes to record information on the information recording medium is shortened and the information transfer rate is increased, the time for crystallization also becomes short. Therefore, also in order to realize phase-changing information recording media for high transfer rates, it is again necessary to make the recording layer from a material with better crystallization capability or to provide a layer with a high crystallization enhancing effect adjacent to the recording layer.
Conventionally, in order to address this problem, and realize a medium with high capacity and suitable for high transfer rates, a material with high crystallization capability is used for the recording layer, and interface layers made of a nitride including Ge, as in the 4.7 GB DVD-RAM, are arranged on both sides of the recording layer.
However, when using a material in which the crystallization capability is increased in order to improve the crystallization speed of the optical phase-changing information recording medium, then the amorphous phase becomes difficult to form in particular in rewritable optical phase-changing information recording media. Therefore, it becomes necessary to heat the recording layer to a higher temperature, widen the melting region of the recording layer, and quickly cool the recording layer. Thus, a higher energy (laser power) becomes necessary to record information, and there is the problem that the recording sensitivity decreases. Moreover, when interface layers made of a nitride including Ge are used as in the conventional case, then there is the problem that the interface layers may be damaged by the heat generated in the recording layer by applying a large energy, considerably deteriorating the repeated rewriting properties.
Furthermore, since the thermal conductivity of nitrides including Ge is high, heat tends to diffuse in particular when the interface layer is thick. Also due to this reason, there is the problem that the recording sensitivity is decreased.
Moreover, when the interface layers are made of a nitride including Ge, then the extinction coefficient of the interface layers becomes large, so that light is absorbed more easily by the interface layers. When more light is absorbed by the interface layers, then there is the problem that the difference between the reflectance in the crystalline phase and the reflectance in the amorphous phase of the optical phase-changing information recording medium becomes small, and the signal intensity decreases. Moreover, more light is absorbed by the interface layers, so that there is the problem that the recording sensitivity decreases even further.